The present invention relates to an ISDN (Integrated Services of Digital Network System) terminal adapter and, in particular, to such a terminal adapter for connecting an analog signal equipment of a four-wire full duplex type to the ISDN line.
In the ISDN system, a digital subscriber line (DSL) extends from an exchanging office to a user's area. The DSL terminates to a network terminal (N.T.). A plurality of terminal equipments are connected to the network terminal and are, therefore, connected to the DSL through the network terminal.
According to CCITT (International Telegraph and Telephone Consultative Communication) recommendations I series, communication information and control signals are transmitted through different channels which are multiplexed in a time division fashion as a time division multiplex (TDM) signal. The TDM signal comprises two communication channels or B channels which are separately called B1 and B2 channels and a single control channel which is called D channel.
In the ISDN system, a virtual circuit is set up on a particular one of B1 and B2 channels through the exchanging office between a calling party of a terminal equipment and a called party of another terminal equipment by a data link establishment and a call control on the D channel in response to a call request from the calling party.
As terminal equipments, there are known a digital telephone set or sets, a facsimile set or sets, a personal computer set or sets, and the like.
In order to enable to access to the ISDN from existing terminal equipments not for ISDN, there are known terminal adapters.
In the prior art, an ISDN terminal adapter of an analog type is used for connecting an existing analog terminal equipment such as a telephone set for an analog line to the ISDN line. The analog port type terminal adapter has one or more analog ports to be connected to one or more telephone sets. Since the analog telephone set is a two-wire type, the known terminal adapter is arranged in a form of the two-wire type for each analog port.
In order to connect an analog signal equipment of a four-wire full duplex type to the ISDN line, two terminal adapters with a single analog port can be used. Alternatively, an adapter with two or more analog ports can be used with two ports being connected to the analog signal equipment. In both cases, two B-channels on the ISDN line can be used corresponding to the two terminal adapters or the two analog ports. Therefore, the analog signal terminal equipment unfortunately results in use of two B-channels for access to the ISDN line at a time.
In a case, for example, a data terminal equipment such as a personal computer performs transmission and reception of data with another data terminal equipment through an analog network, the data terminal equipment is connected through a modulator and demodulator (which is called MODEM) to two analog lines each being made of two wires. One analog line of two wires is for data transmission and the other is for data reception.
The MODEM usually has a line back-up function whereby occurrence of any fault in the currently used analog lines is detected and switching is performed from the fault lines to other analog lines for back-up. That is, the modem has a pair of current ports for data transmission and data reception and another pair of back-up ports for data transmission and data reception.
In order to back up the fault analog lines by the ISDN line, the back-up ports can be connected to the ISDN line through the analog port type ISDN terminal adapter or adapters. Two B-channels on the ISDN line are used for data transmission and data reception, as described above.
It is desired that only one of B-channels is used for the data transmission and data reception even for the analog signal equipment of the four wire full duplex type, the other B-channel can be left for use by other terminal equipments.